We present complementary experimental and computational studies of the relative sizes of equal-length long single-stranded (ss) RNA molecules. Comparison of viral, non-viral, coding and non-coding RNAs of length 2117 nucleotides (nt) shows viral RNA to have among the highest gel-mobilities and smallest hydrodynamic radii in solution. Using graph theoretical tools, we demonstrate that the measured sizes are correlated with the compactness of branching patterns in predicted secondary structure ensembles.